Self-supported Ni-Cu catalyst toward ampere level current in urea electrooxidation via multi-active-site coupling

Abstract

Electrocatalytic urea oxidation reaction (UOR) coupled with hydrogen evolution reaction (HER) to construct urea electrolytic cells is an effective method to achieve low energy consumption for hydrogen production. Two-dimensional Ni(OH)₂ has been widely studied in the field of UOR, however, the development of an ampere-level-current UOR catalyst with fast kinetics is still a challenge. Herein, the self-supporting electrode CF/CNM-S with Cu₂S/Mo-doped Ni(OH)₂ was synthesized which integrates multiple structural features including a heterojunction, a super-metastable phase, a sulfidized surface, and a nanoarray architecture. Experiments combined with theoretical study revealed that using a foam copper derivative as the carrier effectively enhanced the loading and dispersion of Ni(OH)₂ nanosheets, while the nanoarray architecture significantly improved mass transport and diffusion. During the UOR process, the molybdenum-doped NiOOH and CuOOH species generated via the self-reconstruction of CF/CNM-S were identified as the true active phases. The resulting hierarchical heterostructure enabled fine-tuning of the surface electronic states, and the super-metastable state induced by molybdenum doping regulated the catalyst's activation capability towards urea molecules, and facilitated the cleavage of C-N bonds and the oxidation of CO intermediates. Consequently, the as-prepared CF/CNM-S electrode achieved a UOR current density of 1006 mA cm⁻² at 1.8 V (vs. RHE) in an electrolyte containing 3 M KOH and 3 M urea.